The work in this ticket is to 1) write a test that runs an aggregation with explain("executionStats") containing a $group on mongos and 2) verify that the explain output contains memory metrics from the shards and mongos (maxUsedMemBytes in the fields)

SERVER-102384 will add memory metrics to the explain execution stats output for the standalone case.

After SERVER-102382 goes in, we should make sure that when explain("executionStats") is run on a mongos, it should reflect local memory use on a mongos process, not the total memory rolled up from shards. When explain execution stats is run in a sharded case, the "shards" portion of the output should have the memory statistics corresponding to their local memory use